This invention relates to an improved process for bleaching cellulose pulp. In the production of bleached pulp, brown stock is subjected to a series of sequential bleaching and extraction treatments. A variety of bleaching sequences are employed in the industry. Bleaching may be accomplished using chlorine, chlorine dioxide, or mixtures thereof, under acid conditions, or, alternatively, by using oxygen, ozone, peroxide, or hypochlorite under alkaline conditions. Among the most common of these are the chlorine-based processes including the well known CEDED bleaching process and its variants, including the CEHDED bleaching sequence. In the first stage of these processes (bleaching), the pulp is treated typically with chlorine as a gas, as an aqueous gas bubble suspension, or as a dissolved chlorine solution (chlorination) under acid conditions and is washed. In the second stage, the pulp is extracted, typically with sodium hydroxide, in order to remove a significant portion of the lignin. Extraction of the alkali soluble lignin serves to reduce chemical costs in subsequent stages and improves the finished pulp properties overall. An effective caustic extraction stage, therefore, is very desirable in achieving an acceptable finished product in a cost-effective manner.
Various methods have been suggested for achieving enhanced delignification at an early stage in chlorine-based bleaching processes. For example, U.S. Pat. No. 4,451,332 describes a variant of the caustic extraction stage wherein an oxygen-containing gas is mixed with a chlorinated pulp to form a foam which is extracted. U.S. Pat. No. 4,081,317 describes a process for reducing effluent color which involves extracting the pulp with hypochlorite as an adjunct to, or in place of, the caustic extraction stage.
U.S. Pat. No. 2,147,618 describes a process wherein a pulp is treated with chlorine under acid conditions to produce a chlorinated pulp, and the chlorinated pulp is then rendered alkaline and treated with oxygen to remove coloring material. The patent does not describe the extraction of lignin and impurities in a caustic extraction stage, however. The pulp is washed following the oxygen treatment and is then bleached, preferably with hypochlorite, during which air or oxygen is optionally bubbled through the suspension.
It is an object of the present invention to provide a bleaching process which provides a sequentially bleached and extracted pulp with improved pulp properties. These properties include enhanced extraction stage delignification and final bleached brightness without a significant loss in pulp viscosity, which translates into a reduction in the strength properties of the finished paper product.
While many years ago bleach effluents were not considered to be an environmental threat, improvements in pulping and chemical recovery and an increased awareness of the effects of effluent color and toxicity have led to the realization that bleach plant effluents are now one of the most serious environmental problems in the pulp and paper industry. The primary sources of bleach plant pollutants are the chlorination and extraction stages. Lignin and lignin degradation products contribute color, COD, BOD, and toxicity. Hemicelluloses contribute BOD. Bleaching chemicals contribute to effluent toxicity through residual free chlorine and the formation of chlorinated organics. Resins, terpenes, and extractives contribute odor, taste, and toxicity. The alkali extraction stage is the major generation point of bleach plant effluent loadings due to the dissolution of chlorinated lignins. Because more residual lignin must be removed from softwood pulps, softwood bleaching produces more pollution than hardwood bleaching.
Reduction of the environmental impact of bleaching effluents can be achieved in two ways. Waste streams can be treated outside of the bleach plant before discharge or the quantity and quality of the waste streams can be altered by in-plant process changes. Bleach plant wastes are commonly treated with the total mill effluent, and treatment costs are directly related to total effluent volume. Specific treatment of the limited extraction stage flow rather than the total plant volume is one approach to cost reduction. In-plant process changes can be more economical and may result in improved pulp bleaching. In particular, bleaching sequence modifications which improve the quality of bleach plant effluents are desirable.
Therefore, it is a further object of the present invention to provide a bleaching process modification, i.e., chemically reinforced extraction, which has positive effects on the environmental characteristics of the resulting extraction stage filtrates. Also, it is an object of the present invention to provide a chemically reinforced extraction stage which is inexpensive and easy to implement.